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UNITED STATES PATENT OFFICE OOMPOUNDED RUBBER STOCK Alphonse Pechukas, Akron,

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Chemical Corporation, poration of Delaware Ohio, assignor, by Columbia-Southern Pittsburgh, Pa., a cor- No Drawing. Application June 29, 1950, Serial No. 171,237

Claims.

The present invention relates to a novel composition of matter suitable for use as an improved rubber reinforcing pigment and also to rubber compositions containing such pigment and to the methods of producing these materials.

It is known that silica in finely divided state having a particle size below 0.1 micron is an effective rubber reinforcing pigment.

However,

one of the objections to finely divided silica when incorporated in rubber compositions resides in the fact that the compositions containing cure at a very slow rate.

cured compositions often strength characteristics.

compositions containing such silica Furthermore, the finally exhibit unduly low In addition, rubber finely divided silica tend to exhibit substantial heat build-up when .the cured rubber composition is subjected to dynamic tension.

To a substantial degree, these objectionable characteristics of finely divided silica are avoided when the silica contains a substantial quantity (2 to percent by weight) of free Water, that is water which is capable of being vaporized or driven off from the pigment by heating at a temperature of 105 hours. However, the re C. for a period of 24 quired concentration of free water is not present in most silica pigments and in any event may be thus making control dimcult.

driven off during milling observed in prior silica pigments.

I have found that these undesirable strength properties and heat build-up characteristics of silica may be avoided by a water miscible alkylol ing silica and the alkylol position. Thus, the silic the amine or uncoated ethanol amine or other coating the finely divided silica with amine or by incorporatamine in a rubber coma may be precoated with silica together with trialkylol amine may be compounded with rubber and the various other ,agents compounded with rubber.

Various alkylol amines are purpose. These alkylol miscible and the alkyl radical up to 5 carbon atoms.

suitable for the should be water usually contains Typical alkylol amines amines which have been found to be suitable are mono-, diand tri-methanol amines, mono-, diand triethanol amines, monoamines, mono-, and mono-, diand tri alkylol amines which in diand tri-propanol diand tri-isopropanol amines -butanol amine. Other ay be used are 2-ethylaminoethanol, 2,2 ethyliminodiethanol, 2 dimethylaminoethanol, 2diethylaminoethanol, 2- butylaminoethanol, 2-isobutylaminoethanol, 2- methylaminoethanol, 2,2-methyliminodiethanol, 2 amino-Z-l-propanol, 2 amino-l-butanol, 2- amino 2 ethyl 1,3 propanediol, 2-amino-2- methyl-1,3-propanediol and 2-amino-2-hydroxymethyl-1,3-propanediol. Higher alkylol amines may be used provided such amines are water miscible.

The amount of triethanol amine or other alkylol amine which is used is capable of some variation. Normally, about 2 to 15 percent of the amine based upon the weight of the silica, is found to be suitable. While some departure from this range is possible, it is normally found that excessive amounts of amine have an adverse eifect upon the properties of the ultimate rubber composition whereas lower concentrations of amine are insuificiently efiective.

The use of an alkylol amine as herein contemplated has beenfound to be advantageous in connection with the calcium oxide silica pigment which has been described and claimed in a copending application of Edward M. Allen, Serial No. 63,205, filed December 2, 1948. Such application describes a method of preparing a silica pigment by reacting finely divided precipitated calcium silicatehaving an average ultimate particle size as determined by the electron microscope below about 0.1 micron (normally about 0.025 micron as measured by the electron microscope) with an acid having an anion which forms a water soluble calcium salt. Typical acids which may be used for this purpose include hydrochloric acid, nitric acid, hypochlorous acid, bydrobromic acid, and acetic acid.

Sufficient acid is added to produce a slurry having a pH on the acid side, usually below about l. Thereafter, the pH of the slurry is normally adjusted above 7, normally to about 7 to 8, and the precipitated silica is recovered by decantation and filtration and is dried in an oven at temperatures of to C.

The water content of this product normally is above 4. percent by weight, frequently being as much as 15 to 20 percent of the total weight of the product. Such a pigment contains a substantial quantity, usually Within the range of 2 to 10 percent by weight of the pigment of free water as previously defined. The balance of the water present is in the form of bound. water which is the amount of water driven off from a silica pigment of the type herein described by heating the pigment at ignition temperature, for example, 1000 to 1200 C. until no further water can be removed, minus the amount of free water in the pigment. Bound water which is present in concentrations of about 2 to 10 percent by weight of the pigment, appears to be in chemical combination with the silica.

The resultant silica also contains small amounts of calcium or like alkaline earth metal, depending upon the nature of the silicate used in its production and in general, the alkaline earth metal content, computed as alkaline earth metal oxide normally is present in a range of about /2 to 3 percent by weight of the total pigment. On the anhydrous basis, it will be understood that this range is somewhat higher. Thus the alkaline earth metal oxide content of the silica, computed on the anhydrous pigment ranges from about to percent by weight. The following are typical analyses of silica pigments which may be prepared according to the above mentioned application:

The following is a complete analysis of a silica pigment of the type contemplated which is supplied to the trade on a commercial scale:

Per cent MgO 0.12

F6203 0.166 A1203 0.083 TiOz 0.047 Z1O2 0.014 CrzOa 0.001

Loss at 105 C per cent 6.62.

Loss on ignition do 11.62

1 5.0 grams sample 95 ml. E 2 drops saturated K01 solution Percentages in the above tables are by weight.

As pointed out in the above mentioned application, the presence of the free water in the silica materially increases the rate of cure of rubber compositions in which the silica is incorporated and also increases the tensile and other strengths of the rubber composition. Hence, where the free water content is high, the use of an alkylol amine in conjunction with the silica in a rubber composition is not as necessary although improved results are obtained when the alkylol amine is used in such cases. Where the free water content is low or substantially absent, very poor results are obtained unless the alkylol amine is used.

The rubber compositions herein contemplated may be prepared simply by milling silica, alkylol amine, rubber and rubber vulcanization agents and other conventional compounding agents together in a conventional manner and vulcanizing the resulting product. Concentrations ranging from 5 to 100 volumes of silica per hundred volumes of rubber are found to be suitable. The term rubber as herein contemplated, is intended to include natural rubber and the conventional synthetic rubbers such as butadiene-styrene copolymers known as GR-S rubber which are copolymers of 10 to 60 percent by weight of styrene and 90 to 40 percent by weight of butadiene, butadiene-acrylonitrile copolymers derived by oopolymerization of 40 to 90 percent by weight of butadiene, 60 to 10 percent of acrylonitrile, neoprene rubber, isobutylene polymers and copolymers of isobutylene with 0.01 to 4.0 percent by weight of isoprene (based upon the weight of isobutylene) or other elastomers prepared by polymerization of butadiene1,3,isoprene, piperylene, 2,3-dimethyl butadiene, 2-chlorobutadiene-1,3 or other comparable polymerizable compound alone or in admixture With one or more organic, monomeric or ethylenic compounds including acrylonitrile, isobutylene, vinyl chloride, vinyl acetate, styrene, methyl methacrylate, methyl alphachloroacrylate, methyl acrylate and the like.

According to a further embodiment of this invention, the silica may be precoated with the alklyol amine prior to its incorporation in a rubber composition. This may be effected, for example, by spraying the dry pulverulent silica with an alkylol amine while tumbling the silica or otherwise agitating it to an extent necessary in order to insure thorough mixing of the alkylol amine with the silica. Such precoating offers certain definite advantages. In the first place, it permits preparation of a standardized product which may be added to conventional rubber compounds without the necessity for adjustment of the rubber recipe by a rubber compounder during the rubber compounding. Furthermore the alkylol amine becomes more thoroughly associated with the silica and thus is more effective in combating the disadvantageous properties of the silica than would be the case where the amine simply is added to the rubber compound. The invention will be more fully understood by reference to the following illustrative examples:

EXAMPLE I Streams of aqueous sodium silicate solution containing grams per liter of S102 as NazO (SiO2)3.3c and calcium chloride solution containing 100 grams per liter of 021012 and 30 to 40 grams per liter of sodium chloride were fed directly into the central area of a centrifugal pump at 150 F. The rates of flow were adjusted so that calcium chloride was approximately 10 percent in excess over the stoichiometric quantity required for reaction and that the amount of liquid supplied to the pump was about 25 percent below the output capacity of the pump. In consequence, the solutions were subjected to turbulent intermixing in the pump.

The slurry of calcium silicate thus produced was introduced into a tank and sufficient hydrochloric acid solution containing 28% by weight of HCl was added, with stirring, to reduce the pH of the slurry to 2. Thereupon, sufiicient sodium hydroxide solution containing 40% by Weight of NaOl-I was added to raise the pH of the slurry to 7.5. The precipitated silica was recovered by decantation and filtration and was dried in a oven at an oven temperature of to C. for 12 hours. The free water content of the product was within the range of 3 to 8 percent by weight of the pigment.

ple measured by dryinga portion of the sample at 105 C. for 24' hours and measuring the loss of free water as definedand each sample was compounded in a rubber composition according to the following formula:

GR-S 100 Zinc oxide 5 Sulfur a 3 Phenyl-beta-naphthylamine 1 Benzothiazyl disulfide 1.20 Tetramethyl thiuram disulfide 0.15 Paracoumarone-indene resin 15.00 Triethanol amine 4.9 Silica pigment 58.5

The rubber Compounds so prepared were vulcanized according to standard accepted procedure and the vulcanized rubber products thus obtained were subjected to tests according to standard accepted methods of testing Well established in the lowing table:

Table I G t gensilie, PTear(,i Free Moisture Content Percent by are a mm 5 5 4 weight (based on initial pigmen g i Sffigle t l ii cfi inch uses This data illustrates the combined eifect of an alkylol amine and free water to increase the rate of cure of the rubber composition and further to 55 materially increase the tensile strength of the composition at optimum curing time.

The following tests were performed and demonstrate the results obtained when no alkylol amine Parts by weight 15 rubber art in order to determine the tensile 30 time. The free water content of the conditioned sample was determined by drying at C. ;for 24 hours and the dried pigment and the conditioned sample were compounded in rubber compositions according to the following formula:

Parts by weight GRAS 100.0 Zinc oxide .1 5.0 Sulfur 3.0 Phenyi-beta naphthylamine 1.0 Benzothiazyl disulfide 1.2 Tetramethyl thiuram disulfide 0.15 Paracoumarone-indene resin 15.0 Silica pigment 58.5

These rubber compounds were tested for tensile and tear strength according to standard accepted methods normally used in testing such compositions and the following results were obtained:

Table II Free Moisture Content Cure at Tensile, Tear, Pounds Percent by 280 F., Pounds per per Inch $3 on lmtlal Minutes Square Inch thickness 0.5 180 UndercnredL Undercur'ed.

do 1. Do. 1 5.5 450 100.

It will be noted that the above tests clearly illustrate the advantageous effect of the addition of the triethanolamine. Thus in Table I, itis shown that using 4.9 parts by weight of triethanol amine a satisfactorily cured product was obtained within 20 to 30 minutes even when the free water content of the product was as low as 0.5 percent. In contrast, when no triethanol amine was used, as shown in Table II, samples containing as much as 5.5 percent by weight of free water produced uncured rubber compositions even after 120 minutes. Thus it will be seen that the presence of the triethanol amine greatly increased the rate of cure in this case. Similar results are obtained when monoethanolamine and the other amines set forth in columns 1 and 2 are used in lieu of the triethanol amine.

EXAMPLE II To further demonstrate the effect of triethanol amine, silica prepared as in Example I and containing 5.5 percent by weight of free water and 4.2 percent by weight of bound water was compounded with GRS rubber according to the following recipe, test specimens were prepared and cured and the vulcanized rubber was tested to determine its physical properties according to standard methods. The results obtained were as follows:

Parts by weight was present: 60 GR-S 100.0

Finely divided silica prepared according to Zinc oxide 5.0

the disclosure of the above identified application Sulphur 3.0

was dried in an oven at 105 C. overnight. The Phenyl-beta-naphthylamine 1.0

dried pigment contained 0.5 percent free water. Benzothiazyl disulfide 1.2

A 500 gram portion of this pigment was placed 65 'I'etramethyl thiuram disulfide 0.15

in a pan and conditioned at 77 F. and 50 per- Paracoumarone-indene resin 15.0

cent relative humidity for various periods of Triethanolamine As shown 0 Durometer None 180 240 680 780 42 14 7 EXAMPLE 1 11 Finely divided silica prepared as in Example I is mixed with 3.5 parts by weight (based upon the weight of the pigment) of triethanol amine. This coated product is milled with the rubber composition set forth in Example I, omitting the triethanolamine from the recipe. Results comparable to those set forth in Example I are obtained.

It will be understood that the rubber compositions herein contemplated may contain numerous components in addition to silica and rubber. Thus, as shown in the examples, the composition will oontain the usual vulcanizing compositions including accelerators, plasticizers, carbon black and other pigments, etc. In dealing with GR-S rubber, it has been found advantageous to incorporate 5 to 20 parts by weight of a coumarone'indene resin, preferably a coumarone resin having a melting point of about 100 C.

Although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details shall be regarded as limitations upon the scope of the invention except insofar as such limitations are included in the accompanying claims.

I claim:

1. A method of preparing a rubber composition which comprises mixing rubber, a precipitated silica prepared by reaction of finely divided'alkaline earth metal silicate with an acid which forms a water soluble alkaline earth metal References Cited in the file or this patent UNITED STATES PATENTS Number Name Date "2,047,128 Park July '7, 1936 2,114,123 Heuser Apr. 12, 1938 2,250,192 Cuthbertson July 22, 1941 2,329,322 Baty etal -4.-- Sept. 14, 1943 2,457,970 Baillie Jan. 14, 1949 2,531,396 Carter et al j.. Nov. 28, 1950 2,578,605 Sears et al. Dec."1l. 1951 FOREIGN PATENTS Number Country 1 Date 368,867 Great Britain Mar. 17, 1932 ()THER REFERENCES Synthetic Organic Chemicals by Carbide 8: Carbon Chemicals Corp. Twelfth ed. of 1945, page 78.

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